Transpiration

A series of free Science Lessons for 7th Grade and 8th Grade, KS3 and Checkpoint Science in preparation for GCSE and IGCSE Science.

Transpiration in Plants
Transpiration explains how water moves up the plant against gravity in tubes made of dead xylem cells without the use of a pump.

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf through the stomata. This is called transpiration. As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.

Demonstration of transpiration
1. Prepare a potted plant.
2. Wrap the pot on a plastic bag.
3. Put the plant in a bell jar.
4. This is the experimental set-up A.
5. Prepare a similar set-up with the aerial part of the plant removed.
6. This is the control set-up B.
7. Leave the set-ups in bright light for two hours.
8. Observe any changes on the inner wall of the bell jar.
9. In set-up A, there are liquid droplets formed on the inner wall of the bell jar.
10. In set-up B, the bell jar remains unchanged.
11. Test the liquid in set-up A with a piece of dry cobalt(II) chloride paper. Observe any colour change in the paper. (Water changes cobalt(II) chloride paper from blue to red)
12. Record the results and answer the questions.

Questions
1. Why do the pots need to be enclosed in plastic bags?
2. Why are liquid droplets formed on the inner wall of the bell jar in set-up A?
3. What is the liquid? Where does the liquid come from?

1. The plastic bag prevents direct evaporation from the soil.
2. The liquid is condensed on the inner wall of the bell jar.
3. The liquid is water because the cobalt(II) chloride paper is changed from blue to red. The liquid comes from the water vapour transpired from the leaves of the plant.

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